A Group III nitride semiconductor crystal is conventionally known to be also formed through a technique such as molecular beam epitaxy (MBE). In the case where a Group III nitride semiconductor crystal is formed through MBE, atom vapors of Group III element and nitrogen are required as raw materials. The Group III element, which assumes the form of solid metal, is generally placed in a crucible made of PBN (pyrolytic boron nitride) and heated in the crucible, to thereby generate atomic vapor. In contrast, nitrogen assumes the form of gas under ambient conditions, vapor of nitrogen atoms is generally generated by, for example, decomposing molecular nitrogen gas or ammonia. In one procedure of forming atomic nitrogen vapor through decomposition of molecular nitrogen gas, a nitrogen radical generator which employs an inductively coupled plasma generated by applying high-frequency power to a coil-form electrode is employed. In order to enhance the growth rate of a Group III nitride semiconductor by means of a nitrogen radical generator, the nitrogen radical energy and flux density must be enhanced.
The present inventors developed a radical generator which can generate high-density radicals disclosed in Patent Document 1, and a molecular beam epitaxy (MBE) apparatus disclosed in Patent Document 2. The radical generator disclosed in Patent Document 1 has a plasma-generating tube made of a dielectric material, which is connected to a supply tube for supplying a gas at the downstream end thereof, a coil which is disposed around the outer wall of the plasma-generating tube, for generating an inductively coupled plasma in the plasma-generating tube, and an electrode which is disposed around the outer wall of the plasma-generating tube and more proximal to the supply tube than the coil is, for generating a first capacitively coupled plasma in the plasma-generating tube and adding the first capacitively coupled plasma to the inductively coupled plasma. This apparatus can generate a high-density, plasma having a high energy by injecting the plasma having a high energy generated through capacitive coupling to the high-density plasma generated through inductive coupling.